VNU Journal of Science: Earth and Environmental Sciences, Vol 33, No (2017) 23-28 Direct Measurement of Silver Nanoparticles Suspended in Aqueous Solution by Liquid Electrode Plasma - Atomic Emission Spectrometry Le Van Chieu1,*, Nguyen Hoang Tung2 VNU Project Management Department, 144 Xuan Thuy, Cau Giay, Hanoi, Vietnam Institute of Environmental Technology, Vietnam Academy of Science and Technology (VAST), 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam Received 10 August 2017 Revised 17 August 2017; Accepted 22 September 2017 Abstract: This paper presents a quantitative measurement of silver nanoparticles in aqueous suspension by liquid electrode plasma atomic emission spectrometry (LEP-AES) The dependence of the LEP-AES signal intensity on voltage-pulse height and duration was investigated The detection limit and coefficient of variation (CV) were also measured The CV attained a minimum value of 7% for a pulse height of 1080 V and a pulse duration of ms The detection limit (3σ) of silver nanoparticles by LEPAES, under these optimal conditions, was calculated from a calibration curve to be 0.23 µg/g Keywords: Silver nanoparticle, quantitative measurement, liquid electrode plasma, atomic emission spectrometry Introduction has been demonstrated [2], and that of Cd2+ ions to a precision of 0.3 ppm has been reported In addition, Pb and Cu were also investigated by LEP-AES with the detection limits of and 0.52 µg/L, respectively [3] The principle of the LEPAES is illustrated in figure The quantitative analysis of the different elements can be performed by measuring the intensities of their characteristic emission peaks On the other hand, detecting metallic elements in a solid-state environment is also of interest for its potential for nanoparticle (NP) science applications Various techniques such as atomic absorption spectroscopy (AAS), inductively coupled plasma mass spectrometry (ICP-MS), and inductively coupled plasma atomic emission spectrometry (ICP-AES) can yield quantitative measurements of NPs There is an increasing demand for a compact analysis system that would be capable of measuring the concentration of elements in solution; correspondingly, several miniaturized plasma-based approaches have been reported recently [1-5] Among these, liquid electrode plasma atomic emission spectroscopy (LEPAES) has emerged as a simple and highly sensitive analysis method for detecting elements in aqueous solution [6-9] This handheld device is easy to use The detection of metallic ions such as Na+ and Li+ ions dissolved in nitric acid _ Corresponding author Tel.: 84-904119229 Email: lechieu@vnu.edu.vn https://doi.org/10.25073/2588-1094/vnuees.4123 23 24 L.V Chieu, N.H Tung / VNU Journal of Science: Earth and Environmental Sciences, Vol 33, No (2017) 23-28 (a) (b) (pH 7.4) for diluting the silver standards was prepared from pure salts of Na2HPO4, NaH2PO4, and NaCl (WAKO, Japan) Milli-Q water was used throughout all experiments in this study Depth is 50µm Power supply Pt electrode FI (Top view) 30µm 600µm FI Fluidic flow Narrow part Liquid sample 2.2 Apparatus (Detailed top view) (Side view) (c) + + + + + + Air bubble + M M M M Ag - Ag + + - M M M +Plasma + - + + - Emission + M + M M M - + + Ag Ag + - M M M - M - - Sedimentation of the silver NPs was performed by a ultracentrifuge equipment (TOMY, USA) A quartz microchip (LepiCuveC cuvette) (Micro Emission Ltd., Japan) was used for measuring silver NPs, and optical emissions of this NPs were recorded by a spectrometer (Andor Technology, UK, SR-3031A) and a CCD camera (Andor Technology, UK, Newton) 2.3 Sample preparation Figure Mechanism of emission process The sample suspension is introduced into the narrow part of the chip device, in the absence of air bubbles The platinum electrodes are placed at both ends of the flow channel Voltage pulses are applied to the platinum electrodes (a) The constriction region on the chip is magnified in (b) The principle of the emission by the silver nanoparticles is illustrated in (c) However, LEP-AES is a promising approach for achieving more compact and cost-effective measurements Metallic ions have been successfully detected by liquid electrode plasma; however, the application to metallic NPs has so far not been demonstrated In particular, the essential process of atomization of the NPs into individual metallic atoms has not been investigated to date In this study, the detection limit of silver NPs using LEP-AES was systematically investigated Experiment 2.1 Reagents Silver NP standards, a kind of suspended solution, with various diameters 20, 40, and 60 nm (BBI, United Kingdom), and silver-ion standard solution (KANTO, Japan) were used for studying detection of the silver NPs by LEPAES Phosphate-buffered saline (PBS) solution 1.09 g of Na2HPO4, 0.368 g of NaH2PO4, and g of NaCl were dissolved into 100 ml distilled water to create 0.1 M PBS solution (pH 7.4) A µg/g silver NP suspension (diameter 20 nm), diluted from the initial standard with the PBS solution, was used for optimizing the conditions to detect the silver NPs The experiments, directly detecting silver NPs by LEP-AES, were carried out on the suspensions of µg/g of the silver NPs with diameters to be (corresponding to silver ion solution), 20, 40, and 60 nm A calibration curve with points (0, 0.5, 1, 2, and µg/g) was diluted from a initial 20nm-silver NP standard with the PBS solution 2.4 Experimental setup for direct detection of Ag NPs by LEP-AES The experimental setup for detecting the silver NPs by LEP-AES was schematically shown in Fig 2b The sample solution was carefully spiked into the microchannel with a syringe A voltage was applied across the channel and controlled by the pulsed power source This source supplied pulses of predefined intensities and durations (Fig 2a) The resulting plasma excited the silver atoms to generate emissions, which were then captured by an optical fiber and recorded by a spectrometer L.V Chieu, N.H Tung / VNU Journal of Science: Earth and Environmental Sciences, Vol 33, No (2017) 23-28 (a) silver optical emission intensity was minimum (Fig 4) The optimal pulse height was therefore taken at 1080 V a c Total applied pulses a - pulse height b - pulse duration c - interval time between each pulse 800 (b) Pulse power source Syringe pump Intensity [a.u.] b 25 600 400 200 FI FI Optical fiber 800 900 950 1000 1050 1080 1100 1150 1200 Spectrometer Waste Figure Experimental setup (a) Parameters of the voltage pulses, applied at both ends of the flow channel, and generated by the pulsed power source (b) Sample suspension spiked into the microchannel with a syringe The optical emission by the silver nanoparticles is captured by the optical fiber and recorded by the spectrometer and the computer Results and discussion 3.1 Investigation of the optimal conditions for detecting the silver NP by LEP-AES Pulse height [V] Figure Optical intensity of the silver nanoparticle emissions as a function of voltage pulse height 80 60 %CV Computer 40 20 800 900 950 1000 1050 1080 1100 1150 1200 Pulse height [V] Pulse height dependency The µg/g silver NP (20 nm) suspension was used to investigate the pulse height dependence Each measurement consisted of ten equal pulses, and lasted ms at ms intervals, with a height of 800, 900, 950, 1000, 1050, 1080, 1100, 1150, and 1200 V Figure shows the average value of seven repeated measurements for each pulse height There was a clear emission peak for silver appearing at a wavelength of 338 nm The increase in silver peak intensities with the applied pulse height is plotted in Fig For the weak pulses (800-1000 V), the emission intensity was very low, whereas at the other extreme pulses (1050-1200 V), it was relatively high Each pulse height creates a temperature to excite emission of the silver NP, therefore the increase of pulse height leads to a temperature rise, resulting in an increase of the emission intensity However, at 1080 V, the relative coefficient of variation (CV) for the Figure Coefficient of variation of the silver optical emission intensity as a function of the pulse height Pulse duration dependency Using the same NP suspension as mentioned above, ten voltage pulses were applied for each measurement with a duration of 3, 4, 5, 6, 7, 8, 9, and 10 ms The time interval between each pulse was ms, and the pulse height was set to 1080 V, and seven measurements were averaged Increasing pulse duration was expected to result in an increase of the silver optical emssion intensity, however the result showed that the optical emission intensity of the silver NP depended non-monotonically on pulse duration (tp) Intensity of the silver NP was increased with increasing of the from to ms However, at the values above ms, intensity of the silver NP was decreased with incresing of the (Fig 5) L.V Chieu, N.H Tung / VNU Journal of Science: Earth and Environmental Sciences, Vol 33, No (2017) 23-28 26 size change of the silver NPs were performed to prove the direct detection ability of the silver NPs by LEP-AES Intensity (a.u) 300 240 180 Deposition of the silver NPs from the solution 120 60 10 Pulse duration [ms] Figure Optical intensity of the silver nanoparticle emissions as a function of the pulse duration (The time interval between pulses is ms) 100 %CV 80 60 40 20 Pulse application time [ms] 10 Figure Coefficient of variation of the silver optical emission intensity as a function of pulse duration The average CV, excluding the maximum and the minimum values of the seven measurements, was calculated for each value (Fig 6), giving an average CV of 22.5% for silver The CV values for the pulse duration of 3, 4, 5, 6, 7, 8, 9, and 10 ms were 93, 25, 23, 11, 7, 13, 18, and 57%, respectively For the value at ms, the optical emission intensity of silver was highest, and the CV was lowest The optimal pulse duration was therefore taken at ms In conclusion, the pulse height and the pulse duration of 1080 V and ms were respectively used for further experimental studies 3.2 Direct detection of the silver NPs by LEPAES Two experiments including sedimentation of the silver NPs from the solution and analysis of The silver NPs in the solution may maintain an equilibrium between the nanoparticle type and the ion type Therefore, to eliminate the case in which the silver spectrum received from LEPAES was only emitted by the ion, the silver NPs were deposited from the solution And after the solution was separated into two parts, named the deposition and the emergent supernatant, for compairing the intensities of the silver optical emission Many reports indicated that the deposition of metallic NPs from solution was usually performed by a centrifugation method Two concentrations of the silver NPs (0.5 and µg/g) with the size of 20 nm were centrifugated at 14,000 rpm/min in 20 °C for 10 minutes The corresponding spectrum intensities were measured by LEP-AES as follows, ten pulses of the height of 1080 V and duration of ms were applied at intervals of ms The results showed that the intensities of the silver NPs in the depositional parts were significantly higher than those in the supernatants for the concentrations of both 0.5 µg/g and µg/g (Fig 7A and B) Therefore the silver NPs deposited by centrifugation were detected by LEP-AES Analysis of size change of the silver NPs by LEP-AES The suspensions of µg/g of the silver NPs with diameters of (solution of silver ion), 20, 40, and 60 nm were demonstrated by LEP-AES For measuring the corresponding spectrum intensities of the silver NPs, the pulse height of 1080 V and the duration of ms were applied at the intervals of ms The data in Fig showed averages of three sets of pulses The emission intensity decreases as the NP diameter increases and the smallest size of the silver NP (diameter of 20 nm) has the highest intensity among all other sizes (expected the silver NP with “zerodiameter”) (Fig 8) L.V Chieu, N.H Tung / VNU Journal of Science: Earth and Environmental Sciences, Vol 33, No (2017) 23-28 70 the NP size also is an evidence for directly detecting the silver NP by LEP-AES Studies about both the deposition of the silver NPs from the solution and dependence of the silver intensity on the NP size confirmed the direct detection of the silver NPs by LEP-AES STD Below Upper (A) 56 42 Intensity [a.u.] 28 14 Calibration curve and the detection limit 500 (B) 400 300 200 100 332 334 336 338 340 Wavelength [nm] Figure Intensities of the silver NPs in both the deposition and the supernatant parts after using centrifugation method, (A) – the silver NP concentration of 0.5 µg/g, (B) – the silver NP concentration of µg/g 1200 Intensity [a.u.] 27 960 720 480 240 For the purpose of calibration, a 10 µg/g of the silver NP standard suspension with diameter 20 nm was diluted in the PBS to form suspension concentrations of 0, 0.5, 1, 2, and µg/g The measuring conditions for each concentration by LEP-AES were performed as follows Ten pulses of the height of 1080 V and duration of ms were applied at intervals of ms To assess the reproducibility, measurements were repeated seven times for each concentration An increase in the silver NP emission intensity with the concentration was observed (Fig 9) The calibration curve is shown in Fig 10 with the correlation coefficient of 0.959 The %CV was calculated as 29.2% for the silver NP concentrations by the silver NP calibration curve The limit of detection (LOD) for the silver NP was estimated by using the equation LOD = 3σ/s, where σ is the standard deviation of the measurement data of blank solution and s is the slope of calibration curve On the basis of Fig 10, the silver NP detection limit was calculated to be 0.23 µg/g 400 0nm 20nm 40nm 60nm Intensity[a.u.] Figure Optical intensity of the silver nanoparticles as a function of diameter A diameter of represents a silver ion solution ppm AgNPs 338nm Size of AgNPs [nm] 0.5 ppm 300 ppm ppm 200 ppm 100 The results can be explained by steric inhibition of the NP sizes When the NP size increases, density of the NP around the plasma environment of LEP-AES may decrease For this reason, the intensity of the silver NP with the coarser size was lower than that with the finer size The dependence of the silver intensity on 334 336 338 340 342 344 346 Wavelength [nm] Figure Optical emission spectra of the silver NPs at each point in the calibration curve (the silver NPs peak measuring at the wavelength of 338 nm) L.V Chieu, N.H Tung / VNU Journal of Science: Earth and Environmental Sciences, Vol 33, No (2017) 23-28 28 Intensity [a.u.] 350 References y = 73.67x + 23.08 R² = 0.959 250 150 50 -50 0.5 1.5 2.5 3.5 Conc of AgNPs [ppm] Figure 10 Calibration curve of the silver nanoparticle intensity Conclusions The silver NP suspensions were directly quantified by LEP-AES with the detection limit of 0.23 µg/g The results also confirmed that the sensitivity of the silver NP depends on both the height and duration of the applied voltage pulses Our results suggest that LEP-AES may be a potential method for measuring other metallic NPs Further investigations on tagging various antibodies with the silver NP are needed for using LEP-AES as a detection technique in biological applications [1] Iiduka A, Morita Y, Tamiya E and Takamura Y, MicroTAS 2004, Vol 1, The Royal Society of Chemistry, Cambridge, 2004, pp 423-425 [2] Banno M, Tamiya E and Takamura Y 2009 Anal Chim Acta 634 L153 [3] Matsumoto H, Iiduka A, Yamamoto T, Tamiya E and Takamura Y, Proceedings of MicroTAS 2005 Conference, Vol 1, Transducer Research Foundation, Boston, 2005, pp 427-429 [4] Kumagai I, Matsumoto H, Yamamoto T, Tamiya E and Takamura Y, Proceedings of MicroTAS 2006 Conference, Vol 1, Tokyo, 2006, pp 497499 [5] Kumai M, Nakayama K, Furusho Y, Yamamoto T and Takamura Y 2009 Bunseki Kagaku 58 L561 [in Japanese] [6] Kagaya S, Nakada S, Inoue Y, Kamichatani W, Yanai H, Saito M, Yamamoto T, Takamura Y and Tohda K 2010 Anal Sci 26 L515 [7] Yamamoto T, Kurotani I, Yamashita A, Kawai J and Imai S 2010 Bunseki Kagaku 59 L1125 [in Japanese] [8] Nakayama K, Yamamoto T, Hata N, Taguchi S and Takamura Y 2011 Bunseki Kagaku 60 L515 [in Japanese] [9] Jo K W, Kim M G, Shin S M and Lee J H 2008 Appl Phys Lett 92 L1503 Đo trực tiếp hạt nano bạc dung dịch phương pháp plasma điện cực lỏng kết nối phổ phát xạ nguyên tử Lê Văn Chiều1, Nguyễn Hoàng Tùng2 Ban quản lý dự án, Đại học Quốc gia Hà Nội, 144 Xuân Thủy, Hà Nội, Việt Nam Viện Công nghệ môi trường, Viện Hàn lâm Khoa học Cơng nghệ Việt Nam (VAST), 18 Hồng Quốc Việt, Cầu Giấy, Hà Nội, Việt Nam Tóm tắt: Bài báo trình bày phép đo định lượng hạt nano bạc dung dịch plasma điện cực lỏng kết nối phổ phát xạ nguyên tử (LEP-AES) Nghiên cứu khảo sát phụ thuộc cường độ tín hiệu LEP-AES vào chiều cao xung thời gian áp xung Giới hạn phát hệ số biến thiên (CV) khảo sát CV chiều cao xung 1080 V đạt giá trị thấp 7% thời gian áp xung ms Giới hạn phát (3σ) hạt nano bạc LEP-AES điều kiện tối ưu tính tốn từ đường chuẩn 0,23 µg/g Từ khóa: Hạt nano bạc, đo định lượng, plasma điện cực lỏng, phổ phát xạ nguyên tử ... + + + Air bubble + M M M M Ag - Ag + + - M M M +Plasma + - + + - Emission + M + M M M - + + Ag Ag + - M M M - M - - Sedimentation of the silver NPs was performed by a ultracentrifuge equipment... pulse duration (tp) Intensity of the silver NP was increased with increasing of the from to ms However, at the values above ms, intensity of the silver NP was decreased with incresing of the (Fig... suspensions of µg/g of the silver NPs with diameters of (solution of silver ion), 20, 40, and 60 nm were demonstrated by LEP-AES For measuring the corresponding spectrum intensities of the silver